Today, integrated circuits can include many standard cells with different functions. For example, standard cells can be logic gates, such as an AND gate, an OR gate, an XOR gate, a NOT gate, a NAND gate, a NOR gate, and an XNOR gate, and combinational logic circuits such as a multiplexer, a flip-flop, an adder, and a counter. Standard cells can be implemented to realize complex integrated circuit functions. When designing an integrated circuit having specific functions, standard cells are selected. Next, designers, or EDA (Electronic Design Automation) or ECAD (Electronic Computer-Aided Design) tools draw out design layouts of the integrated circuit including the selected standard cells and/or non-standard cells. The design layouts are converted to photomasks. Then, semiconductor integrated circuits can be manufactured, when patterns of various layers, defined by photography processes with the photomasks, are transferred to a substrate.
For convenience of integrated circuit design, a library including frequently used standard cells with their corresponding layouts are established. Therefore, when designing an integrated circuit, a designer can select desired standard cells from the library and places the selected standard cells in an automatic placement and routing block, such that a layout of the integrated circuit can be created.